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R. J. Weber, P. H. McMurry, F. L. Eisele, and D. J. Tanner


Atmospheric measurements of expected homogeneous nucleation precursors and aerosols were made at the Mauna Loa Observatory, Hawaii, from 28 June to 27 July 1992. Large molecular clusters and gas phase species including sulfuric acid (H2SO4), methane sulfonic acid (MSA), and OH were measured with a mass spectrometer. Aerosol measurements consisted of ultrafine particle concentrations (∼3–4 nm diameter) and fine particle size distributions (15–500 nm diameter). The altitude of the observatory (average pressure 680 mb) and the presence of mountain–valley winds permitted sampling of both up- and downslope air. Downslope air was found to have the highest concentrations of MSA but low ultrafine concentrations, whereas, upslope air had the highest H2SO4 and ultrafine concentrations. Episodes of substantial increase in ultrafine particles were observed during periods of rapid increase in H2SO4 concentrations. Total aerosol surface area and H2SO4 concentrations had the strongest influence on ultrafine particle concentrations. It is concluded that for the conditions at Mauna Loa, H2SO4 was a vapor precursor of the newly formed particles, and MSA contributed little to new particle formation. Low concentrations of ultrafine particles were ubiquitous in upslope air and may indicate a widespread, low nucleation rate, source of new particles. The data were also used for estimating particle nucleation and growth rates. Measurements taken suggest that nucleation rates exceed values predicted by the classic theory for binary H2SO4–H2O nucleation.

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